1. Field of the Invention
The present invention relates to a printing apparatus that creates dots in the course of a main scan both in a forward direction and in a backward direction and thereby records an image on a printing medium, as well as to a method of such printing and a recording medium to actualize the printing apparatus.
2. Description of the Related Art
Color printers with a head that can spout inks of several different colors are widely used as an output device of a computer system to print a multi-color image that has been processed by a computer. One proposed technique for such printers creates dots in the course of not only a forward movement but a backward movement of a head for the main scan, which moves the head forward and backward relative to a printing medium. The dot creation by this technique is hereinafter referred to as the dual-way printing.
One procedure of the dual-way printing creates part of the dots included in one raster line with one nozzle in the course of a forward movement of the head, while creating the remaining dots of the raster line with another nozzle in the course of a backward movement of the head. Namely all the dots included in one raster line are completed by the forward and backward movements of the head. This recording method, which records one raster line with different nozzles, disperses the positional displacement of the dots due to mechanical errors in the manufacture of nozzles and thereby improves the picture quality.
Another procedure of the dual-way printing creates all the dots included in one raster line in the course of a forward movement of the head, while creating all the dots included in another raster line in the course of a backward movement of the head. Compared with the single-way printing, this recording method doubles the efficiency of dot creation and thereby enhances the printing speed.
Another proposed technique for the conventional printer having only the binary tone expression (on-off) for each dot enhances the number of tones expressible for each dot. One structure of this technique provides two inks of different densities for an identical hue (see, for example, JAPANESE PATENT LAID-OPEN GAZETTE No. 8-209232). Another structure of this technique enables creation of several different types of dots having different diameters (see, for example, JAPANESE PATENT LAID-OPEN GAZETTE No. 59-201864). These procedures enrich the tone expression of the printer and improve the picture quality of a resulting image.
With an improvement in picture quality of the resulting image output from the printer, it is highly demanded to make the high picture quality compatible with the high printing speed. In order to fulfill these requirements, the inventors of the present invention tried to apply the dual-way printing that enhances the printing speed for the printer that can create dots of different diameters. The dual-way printing may, however, cause the positions of the dots created in the course of a backward movement of the head to be deviated from the expected positions based on the positions of the dots created in the course of a forward movement of the head. This results in non-allowable deterioration of the picture quality. Such positional misalignment of the dots is ascribed to a backlash or play required for the driving mechanism of the printer as well as to a difference in thickness of paper used as the printing medium.
FIG. 13 shows a state of causing positional misalignment of the dots due to a difference in thickness of paper. Referring to FIG. 13(a), a dot dt11 is created on a sheet of paper PA1 in the forward direction of the main scan, whereas a dot dt12 is created in the backward direction of the main scan to be located adjacent to the dot dt11. A nozzle Nz jets ink droplets Ik11 and Ik12 at the positions shown in FIG. 13(a) by taking into account the speeds in the forward direction and the backward direction of the main scan. These ink droplets Ik11 and Ik12 draw the loci shown in FIG. 13(a) and reach the target positions on the paper PA1 to create the dots dt11 and dt12.
FIG. 13(b) shows the state of dot creation on a sheet of paper PA2 having a greater thickness. The distance between the nozzle Nz and the paper PA2 in FIG. 13(b) is smaller than the distance between the nozzle Nz and the paper PA1 in FIG. 13(a). If the ink droplets are jetted at the identical timings with those of FIG. 13(a) in both the forward direction and the backward direction of the main scan, ink droplets Ik21 and Ik22 draw the loci shown in FIG. 13(b) and hit the paper PA2 to create the dots dt21 and dt22. This causes the dots dt21 and dt22 not to be located adjacent to each other and thereby does not give a desired image. In order to obtain a desired image, it is required to delay the ink jet timing in the backward direction of the main scan. Since the difference in thickness of paper may cause the positional misalignment of the dots in the main scanning direction in the case of dual-way printing, it is difficult to completely eliminate the possibility of positional misalignment by the fine regulation at the time of shipment of the printer.
FIG. 14 shows an example of dot creation by a head with a plurality of nozzles in the conventional dual-way printing. In the left part of FIG. 14, both the symbols, the circle O and the square xe2x96xa1, with numerals included therein denote an identical nozzle array. The circles O represent the positions of the respective nozzles in the course of a forward movement of the head, whereas the squares xe2x96xa1 represent the positions of the respective nozzles in the course of a backward movement of the head. The numerals are assigned to the respective nozzles included in the nozzle array, for convenience of explanation. Each of the symbols P1, P2, . . . affixed to the nozzle array represents the number of times of the main scan. After each main scan, a sub-scan is carried out to feed the sheet of paper by a fixed amount of 5 raster lines. A desired image is recorded by creating all the dots in raster lines by the respective main scans.
The right part of FIG. 14 shows dots recorded by the scans of the head. This example forms a vertical ruled line having the width of 2 dots in the main scanning direction. The vertical line is generally formed with dots of a large diameter (hereinafter referred to as large dots), in order to emphasize the contrast with the background. The circles O and the squares xe2x96xa1 represent the dots created in the course of the forward movement and the backward movement of the head, respectively. Raster lines formed in the forward direction of the main scan and raster lines formed in the backward direction of the main scan are arranged alternately. In the example of FIG. 14, the nozzle array includes 13 nozzles and the nozzle pitch in the sub-scanning direction is four times the recording pitch of the image.
As discussed previously with the drawing of FIG. 13, in the case of dual-way printing, the positions of the dots created in the course of a backward movement of the head may be deviated from the positions of the dots created in the course of a forward movement of the head in the main scanning direction. If the dots created in the backward direction of the main scan are displaced rightward relative to the dots created in the forward direction of the main scan, the resulting image is a vertical line where the positions of the dots are periodically changed by every raster line as shown in FIG. 14. This causes the vertical line to be visually recognized as envelopes 11 and 12. Namely the resulting image is recognized not as a straight line but as a wavy curve. The vision of the human is extremely sensitive to such positional misalignment, especially in the vertical direction. Even the displacement of less than one dot as shown in FIG. 14 is recognizable with naked eye. Such positional misalignment is thus not negligible to attain the high picture quality.
FIG. 15 shows the state of dual-way printing that creates the dots of the respective raster lines in the course of both the forward movement and the backward movement of the head to form the vertical ruled line shown in FIG. 14. In this example, part of the dots included in each raster line are created in the course of a forward movement of the head, whereas the remaining dots are created in the course of a backward movement of the head (this technique of dual-way printing is hereinafter referred to as the singling method). The symbols in FIG. 15 have the same meanings as those of FIG. 14. In this case, each raster line is formed by two passes of the main scan in the forward direction and in the backward direction. The sub-scan feeds the paper by 8 raster lines after the forward movement of the head and by 5 raster lines after the backward movement of the head.
If the dots created in the backward direction of the main scan are displaced rightward relative to the dots created in the forward direction of the main scan, the resulting image has the width periodically varied by every raster line as shown in FIG. 15. This causes the vertical line to be visually recognized as envelopes 13 and 14. Namely the image is recognized not as a straight line of a fixed width but as a wavy curve of a periodically varying width.
Such deterioration of the picture quality is also observed in the case where an image is recorded with dots of a small diameter (hereinafter referred to as small dots). FIG. 16 shows an example where small dots are homogeneously dispersed in a certain area. Like the example of FIG. 15, the dual-way printing of the singling method is adopted to record an image in the example of FIG. 16. If the dots created in the backward direction of the main scan are displaced rightward relative to the dots created in the forward direction of the main scan, the dot interval in the main scanning direction is varied in each raster line as shown in FIG. 16. The hatched portions in FIG. 16 have the narrower dot interval and are thereby recognized as dark parts. This damages the homogeneous dispersion of the dots and causes a pattern of varying density to be recognized visually. The image filled with only small dots is often a relatively low tone area where unevenness of dots is conspicuous. The pattern of varying density due to the unevenness of dots is thus not negligible in these areas to attain the high picture quality.
The positional misalignment of the dots in the main scanning direction in the case of dual-way printing is a known problem. The positional misalignment significantly lowers the picture quality to the non-allowable level in the printer that has excellent tone expression and gives an image of high picture quality. The vision of human is extremely sensitive to the phenomenon that causes a vertical ruled line to be visually recognized as a wavy curve (see FIGS. 14 and 15) and to the pattern of varying density in a low-tone area (see FIG. 16). The deterioration of picture quality is especially not allowed in these cases.
The object of the present invention is thus to provide a technique that prevents deterioration of the picture quality due to the positional misalignment of the dots in the main scanning direction, which are created in the course of a forward movement and a backward movement of a head, in a printing apparatus enabling dual-way printing and that allows the high picture quality to be compatible with the high-speed printing.
At least part of the above and the other related objects is realized by a printing apparatus that carries out a main scan, which moves a head forward and backward relative to a printing medium, and creates a plurality of dots on the printing medium according to input image data, thereby printing an image. The printing apparatus includes: the head that enables creation of at least two different types of dots having different densities per unit area; a storage unit that stores a predetermined relationship between the at least two different types of dots and raster-forming directions of the main scan, wherein the raster-forming directions include a forward direction and a backward direction of the main scan that respectively correspond to the forward movement and the backward movement of the head, the predetermined relationship causing each of the raster-forming directions of the main scan to be mapped to at least one type of dot, wherein there is at least one type of dot created only in either one of the forward direction and the backward direction of the main scan; a first creation unit that drives the head in the forward direction to create the type of dot mapped to the forward direction of the main scan, based on the predetermined relationship; and a second creation unit that drives the head in the backward direction to create the type of dot mapped to the backward direction of the main scan, based on the predetermined relationship.
In the printing apparatus of the present invention, the head can create the at least two different types of dots having different densities per unit area, wherein the raster-forming direction of the main scan is preset for each type of dot. The predetermined relationship between the respective types of dots and the raster-forming directions of the main scan, which is stored in advance, causes each of the forward direction and the backward direction of the main scan to be mapped to at least one type of dot and ensures the existence of at least one type of dot created only in either one of the forward direction and the backward direction of the main scan.
The at least two different types of dots having different densities per unit area may be dots created with inks having different densities with respect to an identical hue or dots of different diameters. These two arrangements may be combined to vary the density of the dots in several stages.
The expression that xe2x80x98the predetermined relationship causes each of the forward direction and the backward direction of the main scan to be mapped to at least one type of dotxe2x80x99 does not include the case in which all the different types of dots are created only in either one of the forward direction and the backward direction of the main scan. This is because the object of the present invention is to solve the problems in the dual-way printing.
The expression that xe2x80x98the predetermined relationship ensures the existence of at least one type of dot created only in either one of the forward direction and the backward direction of the main scanxe2x80x99 does not include the case in which, if there are three different types of dots, A, B, and C, all the types of dots A, B, and C are recorded in both the forward direction and the backward direction of the main scan. It is sufficient that there is at least one type of dot created only in either one of the forward direction and the backward direction of the main scan. It is accordingly not necessary that there are one type of dot created only in the forward direction of the main scan and another type of dot created only in the backward direction of the main scan. By way of example, the above relationship is satisfied in the case where one type of dot A is recorded only in the forward direction of the main scan while the other types of dots B and C are recorded in both the forward direction and the backward direction of the main scan. The above relationship is also satisfied in the case where no type of dot is recorded in both the forward direction and the backward direction of the main scan, for example, in the case where the two types of dots A and B are recorded in the forward direction of the main scan while the other type of dot C is recorded in the backward direction of the main scan.
The following briefly describes the dots having the different densities per unit area. FIG. 17 shows an example of using three different types of dots having different densities per unit area (hereinafter referred to as the xe2x80x98dark dotsxe2x80x99, xe2x80x98medium dotsxe2x80x99, and xe2x80x98light dotsxe2x80x99 in a descending order of the density) according to the image data. In the printing apparatus that creates dots to record an image, the tone expression is implemented by varying the dot recording ratio in a solid area according to the input tone value. Referring to FIG. 17, the ratio of recording the light dots having the lowest density is varied according to the tone value in a low tone area. With an increase in tone value, the ratio of recording the medium dots having the higher density is increased to realize the tone value, which can not be expressed even by the full recording ratio (100%) of the light dots. In an area of high tone values, the dark dots having the highest density are mainly recorded to realize the tone expression. The relationship between the dot recording ratio and the tone value shown in FIG. 17 is only illustrative and not restrictive in any sense.
As clearly understood from FIG. 17, some tone values are implemented by recording only a single type of dots. The analysis of the cases in which the non-allowable deterioration of the picture quality occurs in the dual-way printing has clarified that such deterioration of the picture quality is apparent in an image filled with only a single type of dots having an identical density per unit area. These include the cases of FIGS. 14 and 15 in which only large dots are used to record a vertical ruled line and the case of FIG. 16 in which only small dots are used to express a low tone area.
The present invention takes into account these aspects discussed above. The printing apparatus of the present invention effectively prevents the positional misalignment of the specific type of dots in the main scanning direction, which are created only in either the forward direction or the backward direction of the main scan, in the process of recording an image. This arrangement thereby prevents the non-allowable deterioration of the picture quality in the dual-way printing. This accordingly improves the picture quality in the dual-way printing and enables the high picture quality to be compatible with the high-speed printing.
In accordance with one preferable application of the printing apparatus, the predetermined relationship further causes each type of dot to be unequivocally mapped to either one of the forward direction and the backward direction of the main scan, as well as causing each of the raster-forming directions of the main scan to be mapped to at least one type of dot.
The printing apparatus of this structure effectively prevents the positional misalignment of any type of dots in the main scanning direction in the process of recording an image. This arrangement thus further improves the picture quality in the dual-way printing.
The expression that xe2x80x98the predetermined relationship causes each type of dot to be unequivocally mapped to either one of the forward direction and the backward direction of the main scanxe2x80x99 means that the direction of printing a certain type of dot is determined to be either the forward direction or the backward direction of the main scan. This does not include the case in which, if there are three types of dots A, B, and C, one type of dot A is recorded in both the forward direction and the backward direction. The one-to-one mapping is, however, not essential. The above relationship is satisfied, for example, in the case where the two types of dots A and B are recorded in the forward direction of the main scan while the other type of dot C is recorded in the backward direction of the main scan.
In accordance with another preferable application of the printing apparatus, the at least two different types of dots may be at least two different types of dots having different dot diameters.
Since the dots of different diameters are generally created by the same head, the dual-way printing is significantly advantageous to implement the high-speed dot creation. The principle of the present invention is thus effectively applicable to this case.
In the printing apparatus of this structure, it is preferable that the different dot diameters include two dot diameters, and the predetermined relationship stored in the storage unit causes each of the two dot diameters to be one-to-one mapped to the forward direction and the backward direction of the main scan.
In the printing apparatus of this arrangement, the raster-feeding direction of the main scan is one-to-one mapped to the type of the dot. This enables either one of the different types of dots to be necessarily created in the course of the main scan in any direction. This arrangement effectively prevents a significant decrease in efficiency of dot creation. The principle of the present invention is thus effectively applicable to this case.
In accordance with another aspect of the present invention, the printing apparatus further includes: a third creation unit that drives the head in the forward direction to create the at least two different types of dots, irrespective of the predetermined relationship; and a selection unit that causes the first creation unit and the second creation unit to create dots with respect to a predetermined raster data area of the input image data, wherein the predetermined raster data area affects picture quality of the printed image, the selection unit further causing the third creation unit to create dots with respect to a data area other than the predetermined raster data area.
It is especially preferable that the predetermined raster data area of the input image data causes continuous lines to be formed in a direction that crosses the raster-forming directions of the main scan.
As described above, the positional misalignment of the dots in the main scanning direction significantly affects the picture quality in the specific area where only a single type of dots are created. The selection unit causes the first creation unit and the second creation unit to create dots in the specific area, while causing all the dots in each raster line to be created by one main scan in another area. This arrangement improves the efficiency of dot creation and thereby the printing speed. The lines formed in the direction crossing the main scanning direction may be straight lines or curves. In the case of the straight lines, these lines may cross the main scanning direction at any arbitrary angle.
The present invention is also directed to a method of creating a plurality of dots on a printing medium according to input image data with a head and thereby printing an image, the head enabling at least two different types of dots having different densities per unit area to be created in the course of a forward movement and a backward movement of the head relative to the printing medium. The method includes the steps of: (a) determining a type of dot to be created in each of raster-forming directions of a main scan, based on a predetermined relationship between the at least two different types of dots and the raster-forming directions of the main scan, wherein the raster-forming directions include a forward direction and a backward direction of the main scan that respectively correspond to the forward movement and the backward movement of the head, the predetermined relationship causing each of the raster-forming directions of the main scan to be mapped to at least one type of dot, wherein there is at least one type of dot created only in either one of the forward direction and the backward direction of the main scan; (b) driving the head in the forward direction to create the type of dot mapped to the forward direction of the main scan, based on the predetermined relationship; and (c) driving the head in the backward direction to create the type of dot mapped to the backward direction of the main scan, based on the predetermined relationship.
In accordance with one preferable application of the method, the predetermined relationship further causes each type of dot to be unequivocally mapped to either one of the forward direction and the backward direction of the main scan, as well as causing each of the raster-forming directions of the main scan to be mapped to at least one type of dot.
In accordance with another preferable application of the method, the at least two different types of dots may be at least two different types of dots having different dot diameters.
This method exerts the same effects as those of the printing apparatus discussed above and enables an image of a high picture quality to be printed at a high speed.
In the printing apparatus of the present invention discussed above, a computer may carry out the control operations of the head for recording the dots according to a program. Another application of the present invention is accordingly a recording medium for recording the program.
The present invention is thus directed to a recording medium, in which a program for causing a printing apparatus to create a plurality of dots on a printing medium according to input image data and thereby print an image is recorded in a computer readable manner, wherein the plurality of dots include at least two different types of dots. The program causing a computer to carry out the functions of: storing a predetermined relationship between the at least two different types of dots and raster-forming directions of a main scan, wherein the raster-forming directions include a forward direction and a backward direction of the main scan, the predetermined relationship causing each of the raster-forming directions of the main scan to be mapped to at least one type of dot, wherein there is at least one type of dot created only in either one of the forward direction and the backward direction of the main scan; and causing the printing apparatus to create each type of dot in the raster-forming direction of the main scan mapped to the type of dot.
In accordance with one preferable application of the recording medium, the predetermined relationship further causes each type of dot to be unequivocally mapped to either one of the forward direction and the backward direction of the main scan, as well as causing each of the raster-forming directions of the main scan to be mapped to at least one type of dot.
The at least two different types of dots may be at least two different types of dots having different dot diameters.
The computer executes the program recorded in the recording medium, so as to actualize the printing apparatus of the present invention discussed above.
Available examples of the recording media include flexible disks, CD-ROMS, magneto-optic discs, IC cards, ROM cartridges, punched cards, prints with barcodes or other codes printed thereon, internal storage devices (memories like a RAM and a ROM) and external storage devices of the computer, and a variety of other computer readable media. Still another application is a program supply apparatus that supplies a computer program, which causes the computer to actualize the above functions, to the computer via a communications path.